This nation still fighting the flu, and facing another warning tonight. Top scientists say a new strain of a powerful norovirus is now racing person to person across the country. It's a severe stomach flu easy to get, hard to prevent. Abc's chief medical editor, an expert on viruss and epidemics tells us what's new tonight. Reporter: Tonight, it is being called a "superbug." We now know this new strain of norovirus, which makes people violently ill is now ripping across the coury. First identified in australia, it is now ripping across the country. In an average year, about 21 million americans get norovirus, with those classic stomach flu symptoms. 800 will die. It comes on very suddenly, within hours of being exposed, and today, since no one has immunity to this new strain, many more of us, perhaps 50% more, could become violently ill. It's like a fary, fast moving and well designed. Compared to the flu. Flu is spread mostly in the air, by coughs and sneezes. You need to breathe in as many as 1,000 virus particles to get sick. Norovirus? Just 18 particles can do it. Extremely contagious. That means if I'm riding on an escalator and put my hand here, and someone with norovirus put their hand here, two weeks ago, I've just picked it up. That's right, on hard surfaces, revolving doors, doorknobs, coffeemakers, regular flu virus can last two to eight hours. But norovirus could be there, and infectious, for weeks. Cleaning your home, regular detergents don't work. You have to use bleach. You need to get norovirus off your hands before it gets to your stomach. Good old soap and water, over and over again. Now, to be clear, most people are going to have a horrible two to three days and then feel better. The key is to drink plenty of liquids. As we mentioned at the top of the show, hand sanitizers that work for the flu, don't work for norovirus. Anything else you can do? The key is prevention. If you have this disguise, you shouldn't be preparing food or near other people. You got to wait for it to run its course. All right. Thanks so much. Now, we move to another big

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